The present invention relates to a method and apparatus of forming and utilizing defects formed in ionic crystal.
The use of color centers in ionic crystals has been known for some time. A color center laser, for example, is a known light source that operates on a basis of random defects formed in an ionic crystal. See U.S. Pat. No. 5,889,804 entitled, xe2x80x9cArtificial Color Center Light Sourcexe2x80x9d which issued on Mar. 3, 1999 to Y. Takiguchi. In that patent there is described a color center light source where a color center is formed artificially. A predetermined single atom is removed from the surface of a defect-free ionic crystal so as to form a lattice defect. Optical transition of the defect is utilized so that it functions as a light source. In the past, these color centers have been formed by methods such as exposing the crystals to gamma radiation or heating in the presence of excess cations or other impurities. These methods cause anions to be displaced from the crystal lattice. The hole left by the cation can then be filled by an excess electron that is attracted to the void due to the positive ions surrounding it. The electron can then be treated as if in a potential well whose size is smaller than the wavelength of the electron; such a well has discrete energy levels which can be predicted quite easily. When an incident photon hits the trapped electron it will be absorbed if the energy of the photon is the same as the difference between the two energy levels of the electron in the well; this will also cause the electron to be excited into the higher energy state. In this way the electron can be used to absorb only select wavelengths of light that correspond to the energy levels in the well. Once the electron is in an excited state, the surrounding crystal will relax, thereby changing the energy gap between the excited and ground states of the potential well. When the electron decays back into the ground state it will emit a photon with different energy and therefore a different wavelength than the incident photon. This is commonly referred to as an F-center type color center, so called because the absorption of discrete wavelengths gives a unique color to the ionic crystal. In general, for most ionic crystals, an F-Center has an absorption peak within the visible light spectrum, however when an excited electron decays back to the ground state it does so over a smaller energy gap and emits light of a longer wavelength. There are other types of color centers such as FA, FB, F2+, and others which can be created through various types of annealing and bombardment by radiation. The other color centers are caused by various other impurities and dislocations present in the crystal and they will each absorb and emit at different areas of the spectrum.
Use of color centers has been employed in the prior art. See for example, the above-noted patent, U.S. Pat. No. 5,889,804. See also U.S. Pat. No. 4,990,322 entitled, xe2x80x9cNACL:OH Color Center Laserxe2x80x9d which issued on Feb. 5, 1991 to C. R. Pollock et al. and is assigned to Cornell. See also U.S. Pat. No. 4,839,009 entitled, xe2x80x9cNACL:ON Color Center Laserxe2x80x9d which issued on Jun. 13, 1989 to C. R. Pollock et al. See U.S. Pat. No. 4,638,485 entitled, xe2x80x9cSolid State Vibrational Lasers Using F-Center/Molecular-Defect Pairs in Alkali Halidesxe2x80x9d which issued on Jan. 20, 1987 to W. Gellermann et al. See also U.S. Pat. No. 5,267,254 entitled, xe2x80x9cColor Center Laser with Transverse Auxiliary Illuminationxe2x80x9d which issued on Nov. 30, 1993 to I. Schneider et al.
In most of these patents the color centers are created throughout the ionic crystal so that the whole crystal can be used to lase light. A notable exception to this is the point light source patent where a scanning electron microscope is used to create a single F-center dislocation to be used as a point light source. In all cases only a single type of ionic crystal is used so that there is only one absorption and emission peak.
It is an object of the present invention to provide an apparatus and a method of forming and utilizing defects formed in the ionic crystals.
In the present invention a thin layer of ionic crystal is grown on a substrate. The crystal could be of any type of ionic crystal such as NaCl or KCl. The crystal could be a pure form of the chosen compound or could contain contaminates which would shift the wavelength of the created color centers. On top of the thin layer, a second thin layer of a different type of ionic crystal is deposited. The second layer, for example, can be LiF or NaF. When these two layers are irradiated with gamma rays, they will each form color centers at the spots which are irradiated. Because of the differences in crystal properties of the two different ionic crystal layers, their color centers will be at different wavelengths. For instance, NaCl absorbs light at a wavelength of 459.6 nm while LiF absorbs light at 248.2 nm. Once the F-center has absorbed light of a certain wavelength, it will eventually decay and emit light at a different higher wavelength. Accordingly the two separate ionic crystals also emit light at different characteristic wavelengths when illuminated at their unique absorption frequencies. Each layer can be made to lase separately. It is important to make sure that the top layer has absorption energy greater than that of the bottom layer. This way the lower light energy of the bottom layers absorption peak will pass through the top layer and be absorbed only by the bottom layer. By selectively exposing different areas of each layer of the crystals to gamma radiation, it is possible to create unique areas in each layer that contain color centers. If the crystal layers are exposed to light at the wavelength characteristic of the absorption of one layer of crystal, that layer""s pattern will be apparent and emit light at its emission wavelength. In any event, if the device is exposed to light of the second layer""s absorption wavelength, then the second pattern will be exposed and light will be emitted at its characteristic emission wavelength. Thus, as one can ascertain, by the utilization of the above-noted invention, two different wavelengths of light can be emitted in a single device.
In the present invention there are two ways in which to create the above-noted pattern of F-centers or color centers on each of the ionic crystals. This will be subsequently explained.